Clofarabine (systematic IUPAC name: 5-(6-amino-2-chloro-purin-9-yl)-4-fluoro-2-(hydroxyl-methyl)oxolan-3-ol) is a purine nucleotide antimetabolite used for the treatment of various types of leukemias, in particular acute lymphoblastic leukemia.
Several methods for the production of clofarabine are currently established in the art. The synthesis routes are typically based on a coupling reaction between the purine derivatives and an arabinofuranosyl fragment. Depending on the method of fluorine atom introduction, these synthesis schemes can be divided into two groups: (i) methods in which a fluorine atom is included in a ribofuranose fragment and introduced in the molecule at the coupling step; (ii) methods in which a fluorine atom is introduced after the coupling step by chemical transformations of the nucleoside obtained previously.
U.S. Pat. No. 5,034,518 discloses the original synthesis scheme for producing clofarabine developed by Montgomery and coworkers. The synthesis route is illustrated in FIG. 1. In brief, a 1-bromo-2-fluoro-sugar 1 is coupled with 2,6-dichloropurine 2, followed by the amination and deprotection steps. This coupling step results in formation of a mixture of α- and β-anomers of product 3. After chromatographic separation, it is possible to isolate the desired β-anomer with a yield of only 32%. The major disadvantages of this method are in fact the low overall yield of the desired product (13% based on 1), a low stability of starting bromide 1 and a challenging purification of coupling reaction product 3 due to the presence of α-isomer.
U.S. Pat. No. 6,949,640 discloses a modification of the above synthesis route involving (i) the use of salts formed by treatment of 2-chloro-6-substituted-purine with strong bases (such as NaH) instead of the NH-forms of purines, and (ii) the use of 2-deoxy-2-fluoro-3,5-di-O-benzoyl-α-D-arabinofuranosyl bromide 4 instead of O-acetyl derivative 1 (this synthesis pathway is illustrated in FIG. 2). These modifications made it possible to increase the total yield of clofarabine to about 40% based on compound 4. However, the synthesis scheme is hampered by the low stability of bromide 4, which is difficult to produce, and the formation of mixture of anomers at the coupling stage, requiring challenging column chromatography for the separation of the desired product.
The further modification of the method of Montgomery and coworkers is disclosed in U.S. Pat. No. 6,680,382 and further illustrated in FIG. 3. This modification involves the use of the potassium salt of 2-chloroadenine 5 obtained by treatment of 2-chloroadenine with t-BuOK instead of the sodium salt of 2,6-dichloropurine, the use of a ternary mixture of solvents (for example, tert-amyl alcohol, CH2Cl2, CH3CN) and addition of CaH2 for the complete removal of water from solvents. The use of low-polarity solvents at the coupling step made it possible to achieve stereoselective proceeding of the reaction (with a ratio of anomer β to α being up to 15:1). In addition, the presence of an amino group in the starting material 2-chloroadenine made it possible to avoid an amination step of the coupling reaction product. As a result, the total yield of purified clofarabine based on fluorinated sugar 4 is about 32%. However, the fluorinated sugar 4 (obtained in four steps from 1-O-acetyl-2,3,5-tri-O-benzoyl-β-D-ribofuranose) is difficult to produce and has a low stability, two still existing disadvantages of this method.
The method disclosed in US patent publication 2012/0010397 relates to the above-referenced second group of methods for preparing clofarabine. It is illustrated in FIG. 4. 2′,3′,5′-Tri-O-benzoyl-2-chloroadenosine 8 is obtained at the first step by coupling sugar 6 and TMS-protected 2-chloroadenine 7. The further debenzoylation with hydrazine hydrate results in the formation of a mixture of 3′,5′- and 2′,5″-di-O-benzoyl derivatives. After isomerization of 2′,5′-di-O-benzoyl derivative into the 3′,5′-derivative 9, triflate 10 is obtained by treatment of compound 9 with triflic anhydride. Triflate 10, after its fluorination and deprotection, results in the final product clofarabine. However, the disadvantages of this method are the large number of isomers (which are difficult to separate) being formed at the coupling step between 6 and 7 (β-N9, α-N7, β-N7 isomers), the use of highly toxic hydrazine hydrate, the overall yield of purified clofarabine being only 15%.
Hence, there is still an ongoing need for improved methods for the synthesis of clofarabine that overcome the limitations of the established synthesis schemes. In particular, there is a need for a synthesis scheme that results in high yield of the product and does not involve the formation of undesired stereoisomers.
Accordingly, it is an object of the present invention to provide such method for the production of clofarabine.